1. Field of the Invention
The present invention relates to a ferromagnetic tunnel junction, a magnetoresistive element (hereafter referred to as MR element) which employs the ferromagnetic tunnel junction as a sensing portion and it also relates to a magnetic head employing this element.
2. Discussion of Background
Magnetoresistive heads (hereafter referred to as MR heads) that utilize the anisotropic magnetoresistance (hereafter referred to as AMR) effect are in commercial production as magnetic heads for high density magnetic recording. However, since an AMR head typically employs an AMR effect film such as NiFe to constitute the magnetic film, its magnetoresistive ratio (hereafter referred to as MR ratio) and sensitivity are low, at approximately 2% and 0.5%/Oe respectively. Therefore, development of magnetoresistive films (hereafter referred to as MR films) with a higher MR ratio and a higher sensitivity is required.
As a technology that will meet this requirement, a new phenomenon, i.e., the giant magnetoresistance effect (hereafter referred to as GMR effect), has come to light in recent years and since it affords a greater MR ratio compared to the AMR effect film in the prior art, much research into this phenomenon has been conducted. In particular, the GMR effect achieved by utilizing a spin valve (SV) film has become the focus of great interest. Since a spin valve film with a film structure constituted of ferromagnetic film/non-magnetic metallic film/ferromagnetic film/antiferromagnetic film, which creates the GMR effect, demonstrates characteristics with a high degree of sensitivity, at 2 to 5%/Oe, it is thought to have potential as a reproduction element in a next generation magnetic head and further research has commenced toward achieving practical utilization thereof.
Apart from the GMR effect, the phenomenon known as the ferromagnetic tunneling effect, whereby a tunneling effect manifests depending upon the relative angles of magnetization of two ferromagnetic films in a junction constituted of a ferromagnetic film/insulating film/ferromagnetic film, is of interest and research into development of an MR element utilizing this phenomena has been in progress. Since a ferromagnetic tunneling effect film provides an extremely high degree of magnetic field sensitivity, it has potential to be adopted as a reproduction magnetic head in ultra high density magnetic recording on the order of 10 Gbit/inch.sup.2. In IEEE Trans. Magn., MAG-18, 707 (1982), S. Maekawa, V. Gafvert et. al. demonstrated theoretically and through experiment that the manifestation of the tunneling effect depends upon the relative angles of magnetization of the two magnetic films in a magnetic film/insulating film/magnetic film junction.
Japanese Unexamined Patent Publication (KOKAI) No. 42417/1992 discloses a magnetic head provided with a ferromagnetic tunneling effect film capable of detecting minute changes in leaked magnetic flux with a higher degree of sensitivity and a higher degree of resolution than an MR head in the prior art and also discloses that the reproduction sensitivity can be further improved by reducing the junction area to reduce the incidence of pinhole formation in the insulating film.
In addition, Japanese Unexamined Patent Publication (KOKAI) No. 103014/1992 discloses a ferromagnetic tunneling effect film that applies a bias magnetic field from an antiferromagnetic film to a magnetic film and a magnetic head employing this ferromagnetic tunneling effect film.
Furthermore, T. Miyazaki, N. Tezuka et. al. report in J. Magn. Magn. Mater. 139 (1995) L231 that an MR ratio of 18% was achieved at room temperature in an Fe/Al.sub.2 O.sub.3 /Fe tunnel junction. In addition, M. Pomerantz, J. C. Sloczewski, E. Spiller et.al. disclose an Fe/a-Carbon/Fe film.
There are a number of problems that have yet to be addressed in the various types of ferromagnetic tunnel junctions that have been disclosed to date.
Publications on the known art such as Japanese Unexamined Patent Publication (KOKAI) No. 42417/1992, for instance, disclose several means for detecting minute changes in a magnetic flux with a high degree of sensitivity by employing an MR element provided with a ferromagnetic tunneling effect film in order to achieve a high, stable output. In one of such means, one of a pair of magnetic films constituting an MR film with a multilayer structure, namely, the magnetic film whose direction of magnetization changes due to leak magnetic flux from the medium, is required to have a reduced anisotropic dispersion angle and to have a single magnetic domain to ensure that the magnetization rotation occurs entirely and at once. To be more specific, a single magnetic domain is achieved by inserting an intermediate film such as BN in the magnetic film.
However, even with a film with a small anisotropic dispersion angle formed in this manner, there is still a problem in that when the film is patterned to a size of several .mu.m to be operated in a high frequency magnetic field at or above several tens of MHz, disturbance in units of microns in the spinning direction occurs at the end portions of the finely patterned film, thereby forming a magnetic domain wall, which, in turn, disrupts the single magnetic domain structure and results in Barkhausen noise and the like.
Next, in regard to the AMR magnetic head and the spin valve GMR magnetic heads in the prior art, a method for preventing Barkhausen noise by forming a magnetic domain control film at the two end portions of the MR film to apply a longitudinal bias has been disclosed (prior art publications: U.S. Pat. No. 5,018,037 and Japanese Examined Patent Publication No. 21166/1996). In these structures, the magnetic domain control films are formed in direct contact with the areas of the two end portions of the entire sensing portion, since as an AMR head or a spin valve GMR head is used with the current being supplied in a direction parallel to the surface of the MR element, no problem is posed in practical use, even if the magnetic domain control films are in contact with the two end portions of the sensing portion.
However, in a ferromagnetic tunnel junction in which a first ferromagnetic film, an insulating film and a second ferromagnetic film are laminated in the vertical direction, a change in magnetoresistance occurs because of a tunnel current flowing in the direction of the lamination. Thus, if bias magnetic layers for magnetic domain control are placed in contact with the overall end portions of the sensing portion as in the prior art, the upper and lower ferromagnetic films, which are separated by the insulating film, become electrically shorted, cutting off the tunnel current and, consequently, no magnetoresistance change is achieved.
In order to adopt a shield type magnetic head in high density recording/reproduction, it is necessary to reduce the distance between the shields at the ABS. In a shield type magnetic head, an insulating film constituted of alumina and so on, for instance, is normally provided in order to maintain insulation between the ferromagnetic tunnel junction and the shield films in addition to the ferromagnetic tunnel junction between the shields.
Consequently, in order to reduce the distance between the shields, the thicknesses of the insulating film and the ferromagnetic tunnel junction must be set as small as possible. However, if the electrode film is made to be exposed at the ABS as disclosed in the publications on the known art, even with the thicknesses of the insulating film and the ferromagnetic tunnel junction reduced, a certain distance is still required between the electrode film and the shield films, constituting an obstacle to reduction of the shield distance and, therefore, it is not suited for high density recording/reproduction.
In the publications on the known art, the area over which the electrode film is provided is large to ensure that all current runs through the ferromagnetic tunnel junction. However, since it is necessary to assure resisting voltage equal to or greater than a specific level between the ferromagnetic tunnel junction and the shield films, if the electrode film is made to be exposed at the ABS taking up a large area, as disclosed in the publications on the known art, dielectric breakdown due to static electricity tends to occur, and therefore it is not desirable.
It is to be noted that the ferromagnetic tunnel junction according to the present invention is the same as the ferromagnetic tunneling effect film described above.